1. Field of the Invention
The present invention relates to a protection circuit in a semiconductor integrated circuit, and more specifically, to a silicon-control-rectifier (SCR) electrostatic discharge (ESD) protection circuit in a semiconductor integrated circuit.
2. Description of Related Art
Progress in semiconductor technology has decreased the dimensions of many semiconductor devices. With this decrease in semiconductor device dimension, has come an increase in the sensitivity of the device to external signals. For example, most complementary metal-oxide-semiconductor (CMOS) devices have very thin gate oxide layers and shallow junctions to reduce the dimensions of the device and increase the device density in a semiconductor chip, thus increasing the product value of the semiconductor integrated circuit. However, these thin gate oxide layers and shallow junctions cannot withstand high stress. When certain external signals, such as an electrostatic stress, are applied to the CMOS devices, the thin oxide layers and the shallow junctions may be damaged, thereby affecting the performance of the CMOS devices. Therefore, in an integrated circuit which consists of CMOS devices, the input portion must be provided with an ESD protection circuit to prevent electrostatic damage. An ESD protection circuit that includes a silicon-control-rectifier (SCR) is considered to have very good electrostatic discharge performance. Since the SCR ESD protection circuit has a low snap-back holding voltage of about 1-5 volts and a low effective resistance of about 1-3 Ohms, it provides a very good discharge condition for the electrostatic current. However, a secondary breakdown effect occurs in the SCR device when the electrostatic current reaches a certain level. This secondary breakdown effect will damage the SCR device, thus affecting the ESD performance.
Although the SCR protection circuit suffers from the above problem, it discharges the ESD current better than other protection circuits. Therefore, it is very important to prevent the occurrence of the secondary breakdown so as to optimize the ESD performance of the SCR ESD protection circuit.
The secondary breakdown effect in a conventional SCR device will now be described and a new design which can abate the effect can then be provided.
Referring to FIG. 1 in which the top view of a conventional lateral SCR (LSCR) is illustrated, the device includes a cathode which consists of a P-type diffusion region 12 and an N-type diffusion region 14 both in a P-type substrate 10, and an anode which consists of a P-type diffusion region 22 and an N-type diffusion region 24 both in an N-type well 20. The cross-sectional view of the device in FIG. 1 along line II--II is illustrated in FIG. 2. When an electrostatic stress occurs in the anode, a electrostatic discharge path can be formed from the diffusion regions 22 and 24 of the anode diffusion regions to the diffusion regions 12 and 14 of the cathode, through the N-type well 20 and the substrate 10. The discharge path is supposed to be a band path in which the current density distributes uniformly. However, because the substrate material is not perfect and may contain defects, certain positions between the N-type well 20 and the substrate 10 are much weaker than other positions. That is, these positions have lower breakdown voltages. Therefore, if the discharge current density becomes high enough to trigger the breakdown mechanism, the discharge current will concentrate at the certain breakdown positions and increase the current density thereof, as shown in FIG. 1. The abrupt increase in current density tends to result in the secondary breakdown. Moreover, since almost all of the electrostatic current flows through the weak positions of the SCR device in an effect known as the thermal run-away, the device is damaged.
There are various forms of SCR devices, including the LSCR as shown in FIG. 1 and FIG. 2, an MLSCR in which an additional diffusion region is formed between the well and the substrate as shown in FIG. 3, and an LVTSCR in which a transistor is formed between the additional diffusion region and the cathode as shown in FIG. 4. Each of these SCR devices suffers from the same thermal run-away problems and secondary breakdown. As mentioned above, the secondary breakdown occurs when a very high current density exists in the SCR device. In other words, if the electrostatic current in the SCR device is uniformly discharged, the probability that a secondary breakdown will occur can be reduced, thus improving the electrostatic discharge performance.